1. Field of the Invention
The present invention relates to a parametric measuring circuit, and more particularly to a parametric measuring circuit for minimizing oscillation effect.
2. Description of the Related Art
Oscillation effect is usually confronted during parametric measurements at input/output ports of integrated circuits. Among known parametric measuring circuits, one of the widely used circuits is the parametric measuring circuit with a NAND tree structure. A NAND tree can be used to measure input signal levels VIL and VIH. The input signal is the external signal received by the input port. The output terminal of the NAND tree and the output terminal of the internal circuit are coupled to multiplexers to form a complete measuring circuit.
FIG. 1a is a conventional parametric measuring circuit with a NAND tree structure. Referring to FIG. 1a, the conventional parametric measuring circuit comprises an input detection circuit 110 and an output selection circuit 120. The external input signal is transmitted to the integrated circuit through the input terminal 130. The input detection circuit 110 of the parametric measuring circuit receives the input signal levels VIL and VIH, and generates the output detection signal 113 to the output selection circuit 120. The input detection circuit 110 is composed of plural NAND gates 111 and 112. The NAND gates are coupled to each other in a tree structure. The last stage NAND gate 112 of the tree structure outputs the detection signal 113. The output selection circuit 120 is composed of plural multiplexers 121–124. Each of these multiplexers 121–124 is coupled between the internal circuit 100 and a corresponding output terminal, and coupled to the input detection signal 113 generated by the input detection circuit 110. An operation mode control signal 125 is applied to the multiplexer 121–124 for the parametric measuring circuit to operate under a normal operation mode or a measuring mode.
When the integrated circuit operates under a normal operational mode, the multiplexers 122 and 124 of the output selection circuit 120 select and transmit the output signals 101 and 102 generated from the internal circuit 100 to the input terminals of the buffers 141 and 142, respectively. Meanwhile, the multiplexers 121 and 123 select and transmit the output control signals 101c and 102c generated from the internal circuit 100 to the output enable terminals of the buffers 141 and 142, respectively. Under the normal operational mode, the integrated circuit receives and transmits signals through the input terminal 130 and the output terminal 150, respectively.
In order to measure the input signal levels VIL and VIH, and the output signal levels VOL and VOH of the integrated circuit, the integrated circuit should operate under a measuring mode. In the measuring mode, the multiplexers 122 and 124 of the output selection circuit 120 select and transmit the detection signal 113 to the input terminals of the buffers 141 and 142, respectively. Meanwhile, the multiplexers 121 and 123 select and transmit the control signal C, such as logic “1,” to the output enable terminals of the buffers 141 and 142, respectively. By adjusting the input voltage of the input terminal 130 and measuring an output voltage of one of the output terminals 150, the input signal levels VIL and VIH and the output signal levels VOL and VOH can be obtained.
When oscillation effect occurs on the parameter measurement, a precise voltage value cannot be measured. For a clear explanation, a block diagram in FIG. 1b represents the circuit in FIG. 1a. 
FIG. 1b is a block diagram of FIG. 1a in a measuring mode. Referring to FIG. 1b, the input circuit 160 represents the input buffers and the input detection circuit 110 in FIG. 1a. The output circuit 170 represents the output buffers and output selection circuit 120 in FIG. 1a. In addition, the voltage-adjustment symbol 180 represents the adjusted input voltage 161 provided in the measuring mode. Under the measuring mode, the input circuit 160 receives the input voltage 161 and outputs the detection signal 162. The output circuit 170 receives and outputs the detection signal 162 generated from the input circuit 160. For a normally operated measuring circuit, the low input voltage level VIL and the high input voltage level VIH can be precisely measured. In the measuring circuit described above, however, after the input terminal of the input circuit 160 receives the input signal, if interrupted by noises in the circuits, feedback routes 171 and 172 between the input circuit 160 and the output circuit 170 are formed due to the power source line VCC and the ground line GND coupled thereto. As a result, oscillation effect occurs between the input circuit 160 and the output circuit 170. The output signal generated from the output circuit 170 thus carries the oscillation effect and is transmitted back to the input circuit 160 through the feedback routes 171 and 172, resulting in inaccurate parameter measurements.
Accordingly, how to avoid oscillation effect created by noises and to enhance the parameter measurement accuracy in the measuring circuit becomes an imperative task to be dealt with.